In recent years, electrically conductive polyaniline compositions comprising polyaniline and dopant have been notable and used in various fields. For example, it is described in Japanese Unexamined Patent Publication No. 3-35516 that a film comprising the above-mentioned electrically conductive polyaniline composition is used as a solid electrolyte film in an aluminum electrolytic capacitor or a tantalum electrolytic capacitor. That is to say, it is described that an electrolytic capacitor superior in frequency characteristics can be obtained by applying a polyaniline solution on dielectric film to form a polyaniline film and then by doping the polyaniline film with a protonic acid. In addition, electrically the conductive polyaniline composition has been studied for practical use in a multitude of fields such as antistatic materials, electromagnetic shielding materials, magnetic recording media, film capacitors and batteries.
It has been also found out that the polyaniline has an eminently high heat resistance by properly selecting dopants. For example, it is described in Japanese Unexamined Patent Publication No. 10-36667 that an electrically conductive polyaniline composition containing an aliphatic monosulfonic acid having a carboxyl group as a dopant exhibits a very high heat resistance and that the decrease of electric conductivity is within one tenth of the initial value even after standing at a temperature of 125° C. for 650 hours.
It has, however, been conventionally pointed out that an electrically conductive polyaniline composition is not necessarily sufficient in water resistance which is important in practical use together with heat resistance. It has been then proposed that a polymer sulfonic acid is used as a dopant. It has been indeed already known as described in Japanese Unexamined Patent Publication No. 3-28229 that an electrically conductive polyaniline composition is provided with improved water resistance by using a polymer sulfonic acid as a dopant compared with the case of using a low-molecular weight sulfonic acid as a dopant.
When a polymer sulfonic acid such as polyvinylsulfonic acid is used as a dopant, it is true that a polyaniline composition obtained is improved in water resistance as compared with the case of using a low-molecular weight sulfonic acid compound; however, after the polyaniline composition is immersed in water for a long period, the decrease of electric conductivity of the composition is inevitable to a certain degree. The degree of the decrease of electric conductivity is approximately 1/400 of the initial value after being immersed in water for 500 hours.